A wireless autonomous device (WAD) is an electronic device that has no on board battery or wired power supply. WADs are powered by receiving radio frequency (RF) energy that is either directed toward them (a directed source) or is ambient and converting the received RF energy into a direct current (DC) voltage. The DC voltage is used to power on-board electronics, such and a microprocessor and/or sensing circuitry, and an RF transmitter which communicates information, such as a sensor reading, to a remote receiver, commonly referred to as a base station. WADs are employed in a number of fields, such as radio frequency identification (RFID) systems (wherein the WADs are radio frequency tags or transponders), security monitoring and remote sensing, among others. WADs are particularly desirable in certain applications as they have essentially an infinite shelf life and do not require wiring because, as described above, they are powered by RF energy transmitted through the air. As described elsewhere herein, the RF energy that is transmitted through the air for powering WADs may be continuous wave RF energy and/or pulsed RF energy.
The transmission of data, such as measurement data, from a WAD to a base station is a major draw on the power of the WAD. In some applications, this power draw represents a major limiting factor. In a situation where a WAD employs a sensor to measure a parameter of interests, the act of conducting a measurement and then transmitting the measured data to the base station is typically triggered by either the receipt of a command transmitted from the base station to the WAD or through some clock or synchronization scheme. The typical measurement recorded by such a WAD is in a digital format of, for example, 8 to 16 bits of information. In other words, the measured data is transmitted by the WAD in the form of an 8 to 16 bit binary word. The WAD frames this digital information and transmits the entire bit profile to the base station. It is this type of transmission that is the fundamental draw of power limiting the range of the passive WAD (as compared to the data transmission phase, a relatively low amount of power is consumed by the WAD when it is not transmitting, including when it is receiving data transmitted from the base station). If the amount of power transmitted can be reduced, the range of a WAD can be greatly extended.